To survive on land, the earliest land plants had to develop mechanisms to tolerate desiccation. Modern seed plants possess an array of morphological features to retain water (such as conductive tissues, waxy layers on leaves, and openings in leaves that can be control to prevent water loss) and have retained desiccation tolerance in only a few specialized structures (e.g., seeds). Present-day bryophytes (mosses), in contrast, lack water transport and retention tissues, presumably like early land plants. As a result, their normal state is at equilibrium with the surrounding air, creating a water-deficit condition that most seed plants could not tolerate.
The plant specific transcription factor ABSCISIC ACID INSENSITIVE3 (ABI3) functions in ABA signaling during seed maturation and germination. It has been reported that ABI3 is evolutionarily conserved in the non-seed plant Physcomitrella patens. Furthermore, both ABA and ABI3 are required for P. patens vegetative tissue to survive desiccation.